The present invention relates to a bidirectional transmission method and system, and more particularly to a bidirectional transmission method and system suitable to attain relatively low speed full-duplex communication and relatively high speed half-duplex communication through one transmission medium.
In the prior art a bidirectional transmission system which uses a two-wire transmission line, an echo canceller system in which a sending signal and a received signal are separated at a transmission/reception terminal, and a time division transmission system in which a transmission time and a reception time are separated are well known. In high speed data transmission, a time division transmission system does not need means for eliminating an echo, and hence a hardware configuration thereof is simpler than that of an echo canceller system. The time division system is described in detail in IEEE Transaction on Communications Com-30, No. 9 (1982), pages 2057 to 2065.
In the above-mentioned prior art technique, full-duplex communication is generally carried out between a data terminal equipment (DTE) and an exchanger. In a half-duplex communcaiton mode, in which data is transmitted in only one of the two transmission directions, only one-half of the transmission capability of the transmission line is utilized.